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Harrisburg, PA—Ten months after Tropical Storm Lee led to record flooding that devastated the Susquehanna Valley, a new PennEnvironment Research & Policy Center report confirms that extreme rainstorms and snowstorms are happening 52 percent more frequently in Pennsylvania since 1948.

“As the old saying goes, when it rains, it pours—especially in recent years as bigger storms have hit Pennsylvania more often,” said Adam Garber, Field Director for PennEnvironment Research & Policy Center. “We need to heed scientists’ warnings that this dangerous trend is linked to global warming, and do everything we can to cut carbon pollution today.”

Based on an analysis of state data from the National Climatic Data Center, the new report found that heavy downpours or snowstorms that used to happen once every 12 months on average in the state now happen every 7.9 months on average. Moreover, the biggest storms are getting bigger. The largest annual storms in Pennsylvania now produce 23 percent more precipitation, on average, than they did 65 years ago.

Scientists have concluded that the rise in the frequency and severity of heavy rainstorms and snowstorms is linked to global warming. Warming increases evaporation and enables the atmosphere to hold more water, providing more fuel for extreme rainstorms and heavy snowstorms.

Garber pointed to the rainstorm that hit the Susquehanna Valley in September of 2011 as an illustration of what more extreme rainstorms and snowstorms could mean for the state. That rainstorm, which led to over 10,000 residents being evacuated, resulted in record-breaking crests of the Susquehanna River. It even required the Governor’s Mansion in Harrisburg to empty out due to flooding.

Other recent extreme precipitation events in Pennsylvania include:

In August of 2011 Hurricane Irene dumped 4.55 inches of rain in only a few hours, led to 400,000 power outages, 400 downed trees, and 7 collapsed buildings in Philadelphia alone.

The snowstorm that hit Pittsburgh in February of, which dumped 21 inches on the area as the fourth largest snowstorm in Pittsburgh, led to suspended public transportation, closed schools and slippery conditions for commuters.

The new PennEnvironment Research & Policy Center report, When It Rains, It Pours: Global Warming and the Increase in Extreme Precipitation from 1948 to 2011, examines trends in the frequency and the total amount of precipitation of extreme rain and snow storms across the contiguous United States from 1948 to 2011. Using data from 3,700 weather stations and a methodology originally developed by scientists at the National Climatic Data Center and the Illinois State Water Survey, the report identifies storms with the greatest 24-hour precipitation totals at each weather station, and analyzes when those storms occurred. The report also examines trends in the amount of precipitation produced by the largest annual storm at each weather station.

Nationally, the report found that storms with extreme precipitation increased in frequency by 30 percent across the contiguous United States from 1948 to 2011. Moreover, the largest annual storms produced 10 percent more precipitation, on average. At the state level, 43 states show a significant trend toward more frequent storms with extreme precipitation, while only one state (Oregon) shows a significant decline.

“The data is clear,” remarked Irina Marinov from the Department of Earth and Environmental Science at the University of Pennsylvania. “Emissions of CO2 and other greenhouse gases are changing the climate and damaging our environment. More extreme weather, to be expected in our warming climate, should be of high concern, not just to specialists in my field, but to every Pennsylvania citizen.”

Key findings for Pennsylvania and the Mid-Atlantic include:

Extreme rainstorms and snowstorms are becoming more frequent. Pennsylvania experienced a 52% percent increase in the frequency of extreme rainstorms and snowstorms from 1948 to 2011. In other words, heavy downpours or snowstorms that happened once every 12 months on average in 1948 now happen every 7.9 months, on average.

Storms with extreme precipitation increased in frequency by 55 percent in the Mid-Atlantic during the period studied. The Mid-Atlantic region ranks 2nd nationwide for the largest increase in the frequency of storms with heavy precipitation.

The biggest rainstorms and snowstorms are getting bigger. The amount of precipitation released by the largest annual storms in Pennsylvania increased by 23 percent from 1948 to 2011.

“Decades of climate research have led us to the inescapable conclusion that fossil-fuel emissions make the atmosphere warmer, moister, and more prone to producing intense precipitations,” added Dr. Najjar. “The good news is that we can minimize the most harmful impacts through emissions reductions, better land-use planning, and improved infrastructure.”

Garber was careful to note that an increase in the frequency and severity of extreme rainstorms does not mean more water will be available for human use. Higher temperatures fuel extreme rainstorms by increasing rates of evaporation. At the same time, however, that evaporation dries soils. Moreover, scientists expect that, as global warming intensifies, longer periods with relatively little precipitation will tend to mark the periods between heavy rainstorms. As a result, droughts are likely to become more frequent and severe in some regions of the United States. Currently, more than half of the lower United States is suffering through prolonged drought, aggravated by the fact that the last six months have been the hottest January-June period on record.

According to the most recent science, the United States must reduce its total global warming emissions by at least 35 percent below 2005 levels by 2020 and by at least 85 percent by 2050 in order to prevent the most devastating consequences of global warming. PennEnvironment Research & Policy Center highlighted two proposals from the Obama administration—carbon pollution and fuel efficiency standards for cars and light trucks through model year 2025, and the first ever carbon pollution standards for new power plants—as critical steps toward meeting these pollution reduction targets.

“How serious this problem gets is largely within our control –but only if we act boldly to reduce the pollution that fuels global warming,” said Garber. “We applaud the Obama administration for their proposals to cut carbon pollution from vehicles and new power plants, and urge them to move forward with finalizing these critical initiatives this year.”